Specter had the nation's top Democrats in his corner after switching to their party last year, and his final ad push featured President Barack Obama saying, "I love Arlen Specter." But Specter's Republican history and an anti-incumbent mood propelled Sestak to an improbable upset.

It was a bipartisan night of rejection for the Washington establishment. Kentucky Republicans struck the first blow by choosing political newcomer Rand Paul as their Senate nominee. Paul, an ophthalmologist and Tea Party favorite, defeated Secretary of State Trey Grayson, who had been championed by Senate Minority Leader Mitch McConnell of Kentucky.

In a Pennsylvania contest that both parties promoted as a precursor to the midterm elections this fall, Republicans suffered a blow to their plans for taking over Congress. Democrat Mark Critz, a top aide to the late Rep. John Murtha, defeated Republican businessman Tim Burns in a special election to finish Murtha's term representing the working-class Johnstown-area district.

In Arkansas, moderate Democratic Sen. Blanche Lincoln was trying to hold off a netroots-fueled primary challenge from Lt. Gov. Bill Halter. Neither cleared 50 percent of the vote Tuesday night, sending the pair into a June 8 runoff election. The winner will face GOP Rep. John Boozman, a rancher and ophthalmologist whose late brother, Fay Boozman, lost to Lincoln in 1998.

Specter was the third incumbent to lose his job this season, following Utah Sen. Bob Bennett's failure to get the GOP nomination in his state and Democratic Rep. Robert Mollohan's primary loss in West Virginia.

Sestak, a second-term congressman who was in the Navy for 31 years, went up against Obama, Vice President Joe Biden, Pennsylvania Gov. Ed Rendell, unions and other Democratic organizations in his underdog bid. The Democratic Senatorial Campaign Committee, which had supported Specter, turned on a dime after Sestak's victory. DSCC Chairman Robert Menendez called Sestak an energetic campaigner with a compelling personal story who "has proven he takes a back seat to nobody when it comes to shaking-up Washington and taking on the establishment."

Jon Delano, political director of KDKA-TV in Pittsburgh, said Sestak goes into the general election "very much of an unconventional outsider Democrat." He added: "Sestak has done this all on his own. He has had no support from the Washington crowd. None whatsoever. This is a victory that he can savor all by himself."

Sestak will face Republican Pat Toomey in November. Toomey, a former congressman and former head of the anti-tax Club for Growth, nearly ousted Specter in the 2004 GOP primary; the prospect of a rematch helped precipitate Specter's switch to the Democratic Party.

Paul, who crushed his opponent in Kentucky, is the son of Rep. Ron Paul, the longtime Texas Republican who ran for president on a platform that included abolishing the Internal Revenue Service and getting out of Iraq. In an interview over the weekend, Rand Paul told Politics Daily's Walter Shapiro that Sarah Palin was a factor in his surge. "For a year, we've been working to define the race as the conservative outsider versus the moderate insider," he said. "She helped cement that dynamic in the race."

On Tuesday night, Paul said his victory is also a huge victory for the Tea Party movement and sends the message "that we're unhappy and that we want things done differently." McConnell, meanwhile, congratulated Paul on an impressive campaign and said that "now Kentucky Republicans will unite in standing against" President Obama's policies.

Democratic National Committee Chairman Tim Kaine wasted no time in labeling the Paul victory a rebuke of GOP leaders in Washington -- and an advantage for Democrats this fall. He called the race "a show of weakness for the minority leader, and in a race that symbolized the fight over the heart and soul of the Republican Party, Rand Paul overcame McConnell's handpicked candidate by a large margin. Unfortunately for Republicans, ordinary Americans are unlikely to be receptive to extreme candidates like Rand Paul in the general election this November."

The outcome provoked at least one conservative call for McConnell's removal, by activist Richard Viguerie. He called Paul's win a "massive repudiation" of McConnell and Republican congressional leaders, especially coming on the heels of their support for defeated Bob Bennett in Utah and initial support for Florida Gov. Charlie Crist (who is now running for the Senate as an independent). "It is clear that many Washington, D.C., GOP leaders are enormously out of touch with the base of the Republican Party, grassroots conservatives," Viguerie wrote on his blog.

Both parties were closely watching the election in Murtha's district for what it would tell them about their prospects this fall. A Burns win would have demonstrated that the "enthusiasm gap" -- poll findings that Republicans are more motivated to vote this year -- was a real phenomenon that can produce victory. Instead Critz won easily in the only district in the country that switched from Democrat John Kerry in the 2004 presidential race to Republican John McCain in 2008. The outcome calls into question the Republican strategy of running this fall against Obama, House Speaker Nancy Pelosi and Democratic overreach.

Our New Approach to Comments

In an effort to encourage the same level of civil dialogue among Politics Daily’s readers that we expect of our writers – a “civilogue,” to use the term coined by PD’s Jeffrey Weiss – we are requiring commenters to use their AOL or AIM screen names to submit a comment, and we are reading all comments before publishing them. Personal attacks (on writers, other readers, Nancy Pelosi, George W. Bush, or anyone at all) and comments that are not productive additions to the conversation will not be published, period, to make room for a discussion among those with ideas to kick around. Please read our Help and Feedback section for more info.